GeeCON 2025: Pavel Lahoda - Mobile-First Security: A Tale of a Greenhouse and a Bunker

## Overview

Pavel Lahoda presents a comprehensive framework for application security centered on the concept of a "security chain"—the idea that security is only as strong as its weakest link. Using metaphors of a greenhouse (vulnerable) versus a hardened bunker (secure), Lahoda argues that most modern applications violate fundamental security principles by centralizing sensitive data, transferring secrets between clients and servers, and outsourcing critical security functions.

## Key Topics

**Security Chain Principles** The talk outlines five stages of application lifecycle requiring security attention: account creation, authorization, device switching, account recovery/access loss, and account closure. Lahoda emphasizes that security must be holistic—adding a padlock to a greenhouse door is meaningless if the walls are glass.

**Account Creation and Identity** Traditional "identity provider" models are fundamentally flawed because identity is created server-side as stored claims, not verified assertions about the user. Instead, Lahoda advocates for self-sovereign identity where cryptographic key pairs are generated on the user's device. The private key never leaves the device; only the public key and signed identity are sent to the server.

**Authorization and Authentication** Password-based systems that transfer credentials over the network violate core security principles. Multi-factor authentication via SMS (using SS7) expands the attack surface without necessarily improving security. The speaker recommends: - Memorable PINs or short passwords that users actually retain - Biometric factors using face recognition via face detection libraries (achieving ~97% accuracy) - FIDO2 standards, though they lack mandatory proof-of-identity factors - Post-quantum cryptography (matrix-based algorithms) to prepare for quantum computing threats

**Device Pairing and Multi-Device Access** When adding a second device, users should enter "pairing mode" and scan QR codes to establish cryptographic relationships. Both devices should maintain equal copies of the identity and key material. For less capable platforms (web apps), security can be proxied through a mobile device acting as an authenticator.

**Account Recovery and Restricted Access** A novel approach involves granting temporary, restricted access similar to leaving keys with a neighbor. The second device receives "restrictions" (a recipe for permitted actions). Authorization updates require multiple factors: a PIN (something known), the device itself (something possessed), and a time window (something timed). The primary device receives push notifications and can approve, postpone, or deny access requests. This time-lock mechanism provides users oversight while enabling secure delegation.

**Account Closure and Data Control** Instead of trusting servers to delete data, encryption should be controlled by the client. Using HKDF (HMAC-based Key Derivation Function) and elliptic curve encryption, clients generate one-time keys derived from a stored seed value. Data is encrypted on the server but inaccessible without the client-derived key. Even if decrypted, access can be staged to prevent server abuse.

## Technical Details

Several standardized technologies enable this architecture: - **FIDO2**: Passwordless authentication via browser APIs and authenticators - **HKDF**: Key derivation based on hash functions (RFC 5869) - **Post-Quantum Cryptography**: Matrix-based algorithms replacing vulnerable elliptic curve methods; implementations available now despite 10+ year timeline cited by financial institutions - **Face Recognition**: Third-party libraries (DeepFace) provide 128-integer vector representations (~97% accuracy) for biometric verification - **Zero-Knowledge Proofs**: Transmit proof of credentials without exposing sensitive data

Mobile devices are central because they offer: - Hardware-backed cryptographic storage (secure enclaves) - Biometric sensors (face, fingerprint—though third-party access is limited on iOS/Android) - Push notifications for real-time authorization flows - Open-source verifiability (F-Droid on Android; deterministic builds on iOS via toolchain)

## Takeaways

1. **Never transfer secrets**: Passwords, tokens, and authentication codes crossing the network undermine security regardless of encryption claims. 2. **Embrace self-sovereign cryptography**: Users should generate and control their own key pairs; private keys must never leave the device. 3. **Use multiple factors strategically**: Rather than relying on a single second factor (like SMS) that expands attack surface, combine what you know (PIN), what you have (device), and timing (time locks). 4. **Control data, not trust**: Even data stored on servers can be protected by keeping decryption keys client-side and requiring client approval for each access. 5. **Mobile-first security is foundational**: Modern smartphones provide cryptographic, biometric, and notification capabilities that enable secure architectures impossible on other platforms. 6. **Plan for quantum threats now**: Post-quantum cryptographic algorithms are standardized and implementable today; waiting for quantum computers is unnecessary.

The speaker is conducting active research in this space and plans to publish implementations under a working name to be announced in the future.